When a colloidal electrolyte is dispersed in an aqueous medium and an electric potential is applied thereto, the charged particles will migrate to the electrode having the opposite polarity. This process is called electrophoresis and forms the basis for the newly developing art of electrophoretic coating. This area has received considerable attention during the past few years, and the bulk of the effort has been directed toward anodic deposition processes. Cathodic deposition is so new that in the year 1970 R. L. Yates was able to say in a text called Electropainting published by Robert Grapper, Limited, on page 13 that "In all electropainting processes hitherto reported, the particles carry a negative charge and migrate towards the anode." However, recent disclosures such as U.S. Pat. Nos. 3,853,803 and 3,862,894 describe cationic systems. The 803 reference teaches the use of an interpolymer of an acrylic acid monomer and amino-alkyl monomer. The 894 reference discusses a nonaqueous dispersion of acrylic polymers which contain a basic nitrogen atom and which will deposit at the cathode.
In addition, efforts to develop the cationic process have been reported in foreign literature. For example, two German patents, Offenlegungsschrift No. 2,237,114 and Offenlegungsschrift No. 2,252,536 disclose such processes; the first of which employs a polymer containing a quaternary ammonium salt which acts as a zwitterion. When placed in an acidic dispersion the zwitterion assumes a positive charge which causes the resin particles to migrate towards the cathode when an external electric field is imposed.
Typically, metallic substrates are coated to decorate and to prevent corrosion; however, in anodic electrophoretic deposition processes, the coating is deposited in an oxidizing environment which activates the metal and may even begin to oxidize it during the coating process. To eliminate this counterproductive aspect of the presently used anodic electrophoretic processes, the efforts which led to the subject invention were directed toward the more desirable cathodic deposition techniques which tend to passivate the surface and reduce the tendency of the metallic substrate to corrode. It is even possible that at the cathode, oxidized portions of the substrate may be electrochemically reduced to the pure metal.